1. Field of the Invention
The present invention relates to electromechanical translation apparatus of the incremental type capable of precise positioning of articles, for example tools, using an extension actuator usually formed of piezoelectric material. This invention has particular application to a linear actuator or positioner for use in a confined space.
2. Prior Art
Modern technologies often require precision positioning devices. Piezoelectric material based stepping actuators are among the best choices for ultra high resolution actuation due to the unique features of the piezoelectric phenomena. Beside sub-nanometre resolution, high stiffness, and long travel range such as offered by the available EXFO Burleigh “Inchworm”™ piezoelectric-based stepping actuator, more and more applications require positioners to have also the capability of handling heavy loads, excellent stability over very long periods, as well as extended lifetime.
In typical incremental electromechanical translation apparatus, such as linear stepping actuators, high resolution stepwise microscopic movement of a movable member is effected by expanding an extension actuator (e.g. a piezoelectric element or stack) while one of its ends is fixed, as by a clamp, relative to a stationary reference and the other end is free to move, then releasing the one end, fixing the other end by another clamp, and allowing the actuator to contract. Repeating this cycle provides very precise step-by-step movement of a movable member.
The prior art shows three types of arrangements for the movable member, clamps and extension actuator, i.e.:
A. In apparatus as used earlier versions of in the Burleigh “Inchworm™” apparatus, shown for example in U.S. Pat. Nos. 3,902,084 to May, Jr. and U.S. Pat. No. 3,902,085 to Bizzigotti, both issued Aug. 16, 1975, the extension actuator is held stationary at or near its central area, and its opposite ends are each attached to one of two clamps which selectively clamp onto a linearly movable member in the form of a shaft. Other U.S. patents showing this basic arrangement are:
U.S. Pat. No. 4,874,979, issued Oct. 17, 1989 to Rapp;
U.S. Pat. No. 5,319,257, issued Jun. 7, 1994 to McIntyre;
U.S. Pat. No. 6,800,984, issued Oct. 5, 2004 to Marth.
B. In another form of apparatus the extension actuator is incorporated in the movable member, along with two selectively operable clamps, so that both the extension actuator and the clamps move linearly relative to a fixed member. Such arrangements are shown, for example, in the following U.S. patents:
U.S. Pat. No. 3,377,489, issued Apr. 9, 1968 to Brisbane;
U.S. Pat. No. 3,684,904, issued Aug. 15, 1972 to Galutva et al.;
U.S. Pat. No. 4,709,183, issued Nov. 24, 1987 to Lange;
U.S. Pat. No. 5,751,090, issued May 12, 1998 to Henderson, and
U.S. Pat. No. 6,380,661, issued Apr. 30, 2002 to Henderson et al.
C. A third arrangement is shown in U.S. Pat. No. 7,045,932, issued May 16, 2006 to Xu et al., and is referred to herein as the “new generation” Burleigh “Inchworm”™ design. Here again, an extension actuator moves linearly with a movable member or shaft, but the clamps are stationary. U.S. Pat. No. 5,034,647, issued Jul. 23, 1991 to Ohtsuka, also shows an arrangement in which the clamps are stationary and the extension actuator moves with a movable member; however this is a rotary mechanism and not a linear device of the type with which the present invention is concerned.
The device of the Xu et al. patent has the advantage over the other prior art linear devices that, since the clamps are stationary, even if these clamps are relatively heavy, they do not impose any inertia forces on the shaft movement, and this allows for rapid operation of the mechanism. The specific design shown in the Xu et al. patent also has the advantages that:                1. Each of the clamps features a power-off holding lock such that each clamp is held closed while power is off;        2. The piezoelectric elements in the system are preloaded. This makes the load on the piezoelectric elements well controlled and the piezoelectric elements are working in compressive mode for better reliability. Since the strength of a piezoelectric ceramic element is much weaker when it is working in tensile mode or shear mode, as compared to compression mode, those former modes are minimized during the motor operation as well as during the power-off holding.        
The development work done by other parties also gained some success with concepts similar to those introduced by EXFO Burleigh as well as with some variation on the actuation design details. An example is the product series NEXLINE (Trademark) actuators from Physik Instrumente (PI) GmbH which were developed recently, as described in U.S. Pat. No. 6,800,984 to Marth.
Although many of the actuation features provided by the NEXLINE actuator are comparable to those of a Burleigh “Inchworm”™ actuator, the reliability of a NEXLINE system is still questionable, particularly when the actuator is used for supporting heavy load, such as holding heavy-weight optical assemblies in the applications of semiconductor industry as well as in large telescope systems. In the case of the NEXLINE design of the aforesaid Marth patent, the piezoelectric elements are directly involved in the support structure which always bears the heavy load, even when the actuator is in the power-off holding state. For a piezoelectric ceramic material, the impact of excessive mechanical load upon the performance and lifetime is as serious as excessive electrical load. Therefore the reliability of a NEXLINE actuator is compromised due to its operation concept. Compared to the “new generation” Burleigh “Inchworm”™ design of the Xu et al. patent, the disadvantage in reliability is obvious, particular with heavy load over a long period of time, although it is also electrically load-free at power-off state.
Another weakness of the NEXLINE actuator is that high shear force on the piezoelectric elements is a fundamental characteristic caused by the operational mechanism of the NEXLINE design. The shear force is a considerable reliability risk since the shear strength of a ceramic material is much weaker than the strength in the compressive mode and, in the multilayer stack case, the relatively weak bonding lines are directly stressed because of the large shear force needed for the operation. The “new generation” Burleigh “Inchworm”™ design is again superior for the long term reliability of the actuator since its piezoelectric elements are working in compressive mode.
From the performance and reliability point of view, the design of the “new generation” Burleigh “Inchworm”™ actuator described in the Xu et al. patent is very good. However, it requires a relatively large mounting space due to the large aspect ratio of the design; the length of the actuator cannot be easily reduced. In some applications, such as for the positioning of optical assemblies in a semiconductor manufacturing system, the allowable space in the motion direction for mounting a linear actuator is limited. A more flexible design is needed for the actuator to be able to be implemented in more applications. As shown in the aforementioned Xu et al. patent, the shaft of the “new generation” Burleigh “Inchworm”™ actuator contains an extension piezoelectric element which divides the shaft into two parts. While one part is clamped, the other part can be moved against the clamped part by controlling the piezoelectric element to realize the extension. By controlling the clamping of these two parts and the extension action, the shaft can be moved linearly in either direction along the axis of the extension action. In Xu et al. the clamps are located beyond opposite ends of the extension element in the motion direction. This makes the length or aspect ratio large relative to the range of travel since in the motion direction the system needs the space to line up three key components, namely clamp-extension-clamp, and a large travel range requires extra length at each end for clamping. The travel range is limited by the distance between two clamps after subtracting the length needed for the extension element and for keeping the structure strong.